1. Field of the Invention
This invention relates to a photodetector for use in light output monitoring devices of semiconductor lasers, receiving devices of optical communication systems, etc.
2. Related Background Art
FIG. 1A is a top view of the structure of a conventional light detecting element, and FIG. 1B is a sectional view along the line X--X' in FIG. 1A. As shown, the conventional light detecting element comprises a first conduction-type semiconductor substrate 1 with a first electrode 8 formed on the underside; a first conduction-type semiconductor crystal layer 2 including a light absorbing layer; and a second conduction-type first region 3 formed in the first conduction-type semiconductor crystal layer 2 by selectively diffusing a dopant. Thus formed is a pin photodiode structure. This pin photodiode structure includes an n-layer (or a p-layer) provided by the semiconductor substrate 1, a p-layer (or an n-layer) provided by a first region 3, and a light detecting region 10 provided by the pn junction (the depletion layer or the i layer). A second electrode 5 is provided on the first region 3 in the semiconductor crystal layer 2. The top of the first region 3 inside the electrode 5 is covered with a reflection preventive film 6, and the top of semiconductor crystal layer 2 outside the electrode 5 is covered with a device protective film 7.
In the semiconductor device of the above-described structure, when an reverse bias is applied, an electric field is generated in the depletion layer. Electrons and holes generated by incident light on a light detecting region 10 are divided respectively to the first conduction-type region 3 and are accelerated. Thus a photocurrent can be taken outside, and an optical signal can be detected.
In the above-described structure of FIG. 1A and 1B, when light is incident on the light detecting region 10, photo-carriers are generated in the depletion layer, and a good response characteristic can be obtained. But when light is incident outside the light detecting region 10, due to a density gradient, the generated carriers are diffused to reach the depletion layer, and are taken out in a photocurrent. The transfer of the diffused carriers is slow. When the carriers reach the light detecting region 10, adversely a tail is generated at the last transition of a light-pulse-responding waveform as shown in FIG. 2.
In using such light detecting element in photodetectors for use in optical communication, etc., a lens 11, such as a spherical lens, a SELFOC lens or others, is disposed at the light incident part of the cap of the package as shown in FIG. 3 so as not to affect the response characteristic. This arrangement enables all the signal light emitted from an optical fiber or others to be focussed to be incident on the light detecting region 3. But this condensation increases an incident light intensity per a unit area of signal light incident on the light detecting region 3, and accordingly more carriers are generated in the depletion layer 10. Resultantly because of the space-charge effect produced by an increase of a carrier density in the depletion layer 10, the intensity of an electric field in the depletion layer 10 is decreased, and a drift rate of the carriers in the depletion layer 10 is lowered. Also tails occur at the falls of light pulse response waveforms. In view of this, the light amount to be incident on the light detecting element 20 has to be limited, and it is a problem that a maximum incident light amount on the semiconductor photodetector cannot be increased. This effect is more conspicuous especially when the reverse bias voltage is low, which makes it difficult to operate the semiconductor photodetectors at low bias voltages.
In controlling a light output of a laser diode to be constant, the light emitted from the rear end surface of the laser diode is detected by a light detecting element, and an operating current of the laser diode is feed-back controlled. But because the light output of the laser diode is so intense that when light is focussed and incident on the light detecting region 3, the space-charge effect occurs, and as described above, the drift of the carriers is increased, and tails occur at the falls of response waveforms. The feed-back control of the laser diode is affected.